• تَطْمَئِنَّ
• تَطْمَئِنُّ
• ٱطْمَأَنَّ
• ٱطْمَأْنَنتُمْ
• ٱطْمَأَنُّوا۟
• مُطْمَئِنّ
• مُّطْمَئِنَّة
• يَطْمَئِنَّ

Appears 12 times in the Quran.

1. 2:260 وَإِذْ قَالَ إِبْرَٰهِۦمُ رَبِّ أَرِنِى كَيْفَ تُحْىِ ٱلْمَوْتَىٰ قَالَ أَوَلَمْ تُؤْمِن قَالَ بَلَىٰ وَلَٰكِن لِّيَطْمَئِنَّ قَلْبِى قَالَ فَخُذْ أَرْبَعَةً مِّنَ ٱلطَّيْرِ فَصُرْهُنَّ إِلَيْكَ ثُمَّ ٱجْعَلْ عَلَىٰ كُلِّ جَبَلٍ مِّنْهُنَّ جُزْءًا ثُمَّ ٱدْعُهُنَّ يَأْتِينَكَ سَعْيًا وَٱعْلَمْ أَنَّ ٱللَّهَ عَزِيزٌ حَكِيمٌ

   And when Ibrahim said: My Lord! show me how Thou givest life to the dead, He said: What! and do you not believe? He said: Yes, but that my heart may be at ease. He said: Then take four of the birds, then train them to follow you, then place on every mountain a part of them, then call them, they will come to you flying; and know that Allah is Mighty, Wise.
   

2. 3:126 وَمَا جَعَلَهُ ٱللَّهُ إِلَّا بُشْرَىٰ لَكُمْ وَلِتَطْمَئِنَّ قُلُوبُكُم بِهِۦ وَمَا ٱلنَّصْرُ إِلَّا مِنْ عِندِ ٱللَّهِ ٱلْعَزِيزِ ٱلْحَكِيمِ

   And Allah did not make it but as good news for you, and that your hearts might be at ease thereby, and victory is only from Allah, the Mighty, the Wise.
   

3. 4:103 فَإِذَا قَضَيْتُمُ ٱلصَّلَوٰةَ فَٱذْكُرُوا۟ ٱللَّهَ قِيَٰمًا وَقُعُودًا وَعَلَىٰ جُنُوبِكُمْ فَإِذَا ٱطْمَأْنَنتُمْ فَأَقِيمُوا۟ ٱلصَّلَوٰةَ إِنَّ ٱلصَّلَوٰةَ كَانَتْ عَلَى ٱلْمُؤْمِنِينَ كِتَٰبًا مَّوْقُوتًا

   Then when you have finished the prayer, remember Allah standing and sitting and reclining; but when you are secure (from danger) keep up prayer; surely prayer is a timed ordinance for the believers.
   

4. 5:113 قَالُوا۟ نُرِيدُ أَن نَّأْكُلَ مِنْهَا وَتَطْمَئِنَّ قُلُوبُنَا وَنَعْلَمَ أَن قَدْ صَدَقْتَنَا وَنَكُونَ عَلَيْهَا مِنَ ٱلشَّٰهِدِينَ

   They said: We desire that we should eat of it and that our hearts should be at rest, and that we may know that you have indeed spoken the truth to us and that we may be of the witnesses to it.
   

5. 8:10 وَمَا جَعَلَهُ ٱللَّهُ إِلَّا بُشْرَىٰ وَلِتَطْمَئِنَّ بِهِۦ قُلُوبُكُمْ وَمَا ٱلنَّصْرُ إِلَّا مِنْ عِندِ ٱللَّهِ إِنَّ ٱللَّهَ عَزِيزٌ حَكِيمٌ

   And Allah only gave it as a good news and that your hearts might be at ease thereby; and victory is only from Allah; surely Allah is Mighty, Wise.
   

6. 10:7 إِنَّ ٱلَّذِينَ لَا يَرْجُونَ لِقَآءَنَا وَرَضُوا۟ بِٱلْحَيَوٰةِ ٱلدُّنْيَا وَٱطْمَأَنُّوا۟ بِهَا وَٱلَّذِينَ هُمْ عَنْ ءَايَٰتِنَا غَٰفِلُونَ

   Surely those who do not hope in Our meeting and are pleased with this world's life and are content with it, and those who are heedless of Our communications:
   

7. 13:28 ٱلَّذِينَ ءَامَنُوا۟ وَتَطْمَئِنُّ قُلُوبُهُم بِذِكْرِ ٱللَّهِ أَلَا بِذِكْرِ ٱللَّهِ تَطْمَئِنُّ ٱلْقُلُوبُ

   Those who believe and whose hearts are set at rest by the remembrance of Allah; now surely by Allah's remembrance are the hearts set at rest.
   

8. 16:106 مَن كَفَرَ بِٱللَّهِ مِنۢ بَعْدِ إِيمَٰنِهِۦٓ إِلَّا مَنْ أُكْرِهَ وَقَلْبُهُۥ مُطْمَئِنٌّۢ بِٱلْإِيمَٰنِ وَلَٰكِن مَّن شَرَحَ بِٱلْكُفْرِ صَدْرًا فَعَلَيْهِمْ غَضَبٌ مِّنَ ٱللَّهِ وَلَهُمْ عَذَابٌ عَظِيمٌ

   He who disbelieves in Allah after his having believed, not he who is compelled while his heart is at rest on account of faith, but he who opens (his) breast to disbelief-- on these is the wrath of Allah, and they shall have a grievous chastisement.
   

9. 16:112 وَضَرَبَ ٱللَّهُ مَثَلًا قَرْيَةً كَانَتْ ءَامِنَةً مُّطْمَئِنَّةً يَأْتِيهَا رِزْقُهَا رَغَدًا مِّن كُلِّ مَكَانٍ فَكَفَرَتْ بِأَنْعُمِ ٱللَّهِ فَأَذَٰقَهَا ٱللَّهُ لِبَاسَ ٱلْجُوعِ وَٱلْخَوْفِ بِمَا كَانُوا۟ يَصْنَعُونَ

   And Allah sets forth a parable: (Consider) a town safe and secure to which its means of subsistence come in abundance from every quarter; but it became ungrateful to Allah's favors, therefore Allah made it to taste the utmost degree of hunger and fear because of what they wrought.
   

10. 17:95 قُل لَّوْ كَانَ فِى ٱلْأَرْضِ مَلَٰٓئِكَةٌ يَمْشُونَ مُطْمَئِنِّينَ لَنَزَّلْنَا عَلَيْهِم مِّنَ ٱلسَّمَآءِ مَلَكًا رَّسُولًا

    Say: Had there been in the earth angels walking about as settlers, We would certainly have sent down to them from the heaven an angel as a messenger.
    

11. 22:11 وَمِنَ ٱلنَّاسِ مَن يَعْبُدُ ٱللَّهَ عَلَىٰ حَرْفٍ فَإِنْ أَصَابَهُۥ خَيْرٌ ٱطْمَأَنَّ بِهِۦ وَإِنْ أَصَابَتْهُ فِتْنَةٌ ٱنقَلَبَ عَلَىٰ وَجْهِهِۦ خَسِرَ ٱلدُّنْيَا وَٱلْءَاخِرَةَ ذَٰلِكَ هُوَ ٱلْخُسْرَانُ ٱلْمُبِينُ

    And among men is he who serves Allah (standing) on the verge, so that if good befalls him he is satisfied therewith, but if a trial afflict him he turns back headlong; he loses this world as well as the hereafter; that is a manifest loss.
    

12. 89:27 يَٰٓأَيَّتُهَا ٱلنَّفْسُ ٱلْمُطْمَئِنَّةُ

    O soul that art at rest!